Prevent removing trailing spaces on certain documents

When writing in Code, it's usually useful if trailing spaces are removed, but when writing in [Pandoc's Markdown](https://pandoc.org/MANUAL.html#pandocs-markdown), two trailing spaces are a symbol for a new line.

It would be very useful if there was a way to disable deleting trailing whitespaces for certain file types, like _.md_.

No files or tests are named. Start by locating the trailing-whitespace removal behavior and how Markdown file types are identified; done means trailing spaces remain in the requested document types while removal continues for other documents.

Written by the indexing model from the issue text.